1. Field of the Invention
The present invention relates to a composition which is advantageously used for optical materials, such as plastic lenses, prisms, substrates of information recording materials and filters, particularly as a material of plastic lenses of glasses.
2. Description of the Related Arts
Plastic materials have widely been used as various optical materials, particularly as lenses of glasses, because of light weight, toughness and easiness of tinting. The properties required for optical materials, particularly for lenses of glasses, are a low specific gravity, optical properties such as large refractive index and Abbe number and physical properties such as high heat resistance and strength. A large refractive index is important to decrease thickness of a lens. A large Abbe number is important to decrease the chromatic aberration of a lens. High heat resistance and strength are important to facilitate fabrication and also from the standpoint of safety.
As conventional materials having a large refractive index, thermosetting optical materials having a thiourethane structure which are obtained by the reaction of a polythiol compound and a polyisocyanate compound have been proposed (Japanese Patent Publication Heisei 4(1992)-58489 and Japanese Patent Application Laid-Open No. Heisei 5(1993)-148340). Technology to obtain a lens by polymerization of an epoxy resin or an episulfide resin with a multi-functional compound have also been proposed in the specifications of Japanese Patent Application Laid-Open No. Heisei 1(1989)-98615, Japanese Patent Application Laid-Open No. Heisei 3(1991)-81320 and International Patent Application Laid-Open No. wo8910575. Of course, optical materials having further large refractive indices are desirable.
A small chromatic aberration is another important property required for an optical material. The larger the Abbe number, the smaller the chromatic aberration. Therefore, a material having a large Abbe number is also desirable. Thus, a material having both large refractive index and large Abbe number is desired.
However, the Abbe number tends to decrease with an increase in the refractive index. Plastic materials obtained from conventional compounds have a maximum Abbe number of about 50 to 55 when the refractive index is 1.50 to 1.55, about 40 when the refractive index is 1.60 and about 31 when the refractive index is 1.66. When the refractive index is forced to increase to 1.70, the Abbe number decreases to 30 or less and the obtained material cannot practically be used.
Moreover, in the case of conventional materials, particularly materials having a thiourethane structure, the molecular weight of a sulfur-containing compound used as the raw material must be increased to achieve a large refractive index and this decreases the crosslinking density. The content of alkyl groups must be increased to achieve a large Abbe number and this decreases the rigidity of molecules of the raw material compounds. As the result, drawbacks such as a decrease in heat resistance arise. In other words, the refractive index of conventional optical materials obtained from episulfide compounds, polythiol compounds and isocyanate compounds can be increased only to a limited degree. Moreover, an increase in the refractive index decreases the Abbe number. Therefore, an advantageous balance between a large refractive index and a large Abbe number cannot be achieved by conventional optical materials.
To solve the above problems, the present inventors discovered a novel sulfur-containing compound having an episulfide structure from which an optical material having a small thickness and a decreased chromatic aberration could be obtained (Japanese Patent Application Nos. Heisei 8(1996)-214631 and Heisei 8(1996)-5797). However, the optical material obtained by curing this sulfur-containing compound by polymerization does not show a sufficient tint performance.